This invention is in the field of free piston Stirling machines and more particularly is directed to an improved free piston Stirling machine of the gamma class which minimizes the dead volume normally associated with the gamma configuration.
In a Stirling machine, a working gas is confined in a working space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do work or to pump heat. Each Stirling machine has at least two pistons, one referred to as a displacer and the other referred to as a power piston and often just as a piston. The reciprocating displacer cyclically shuttles a working gas between the compression space and the expansion space which are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter. The shuttling cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in the entire work space at any instant of time because the expansion and compression spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When most of the working gas is in the compression space, heat is rejected from the gas. When most of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce work so it can function as an engine and if this expansion process temperature is lower than the compression space temperature, then the machine will pump heat from a cold source to a warm heat sink.
Stirling machines can therefore be designed to use the above principles to provide either: (1) an engine having a piston and displacer driven by applying an external source of heat energy to the expansion space and transferring heat away from the compression space and therefore capable of being a prime mover for a mechanical load, or (2) a heat pump having the power piston (and sometimes the displacer) cyclically driven by a prime mover for pumping heat from the expansion space to the compression space and therefore capable of pumping heat energy from a cooler mass to a warmer mass. The heat pump mode permits Stirling machines to be used for cooling an object in thermal connection to its expansion space, including to cryogenic temperatures, or heating an object, such as a home heating heat exchanger, in thermal connection to its compression space. Therefore, the term Stirling “machine” is used to generically include both Stirling engines and Stirling heat pumps, the latter sometimes being referred to a coolers.
Until about 1965, Stirling machines were constructed as kinematically driven machines meaning that the piston and displacer are connected to each other by a mechanical linkage, typically connecting rods and crankshafts. The free piston Stirling machine was then invented by William Beale. In the free piston Stirling machine, the pistons are not connected to a mechanical drive linkage. A free-piston Stirling machine is a thermo-mechanical oscillator and one of its pistons, the displacer, is driven by the working gas pressure variations and differences in spaces or chambers in the machine. The power piston, is either driven by a reciprocating prime mover when the Stirling machine is operated in its heat pumping mode or drives a reciprocating mechanical load when the Stirling machine is operated as an engine.
As well known in the art, there are three principal configurations of Stirling machines. The alpha configuration has at least two pistons in separate cylinders and the expansion space bounded by each piston is connected to a compression space bounded by another piston in another cylinder. These connections are arranged in a series loop connecting the expansion and compression spaces of multiple cylinders. The beta Stirling has a single power piston arranged within the same cylinder as a displacer piston. A gamma Stirling is similar to a beta Stirling but has the power piston mounted in a separate cylinder alongside the displacer piston cylinder.
As is well known, in free-piston Stirling engines and coolers, the displacer and the piston both must be able to freely operate with minimum friction. Since oil or similar lubricants are impractical for use in Stirling machines, non-contact bearings of various types have come to be generally applied. Some researchers use radially stiff flat springs to support the moving parts so as to avoid contact during operation while others have used static gas bearings. All these methods require extremely close tolerances in order to avoid excessive leakage losses and mechanical contact between the moving parts. In the standard displacer-piston beta arrangement, the precision requirements of the displacer and piston compound each other since the displacer rod penetrates the piston. The co-axial alignment of the displacer rod within the piston places additional demands on precision in both displacer and piston and is therefore a strong cost driver.
These problems can be seen in the prior art beta type free piston Stirling machine illustrated in FIG. 1. A hermetically sealed casing 10 has a piston 12 that is reciprocatable in a cylinder 14 and a displacer 16 with a displacer rod 18 that sealingly slides through the piston 12. The end of the displacer rod 18 is connected to a planar spring 20. The work space comprises an expansion space 22 in fluid communication with a compression space 24 through heat exchangers 26 and 28 and a regenerator 30. This illustrates the problem of maintaining the simultaneous alignment of all the interfacing cylindrical surfaces in a manner that has the minimum friction between them but also has sealing engagement between them. All these cylindrical surfaces need to be aligned coaxially and the spaces between them must be small enough to provide a gas seal between them and large enough to minimize friction between them and to make alignment practical.
In the beta arrangement of FIG. 1, each of the reciprocating components is precision aligned in its cylinder. The displacer rod 18 penetrates the piston 12 with a fit requiring concentricity precision along its length with the piston and must therefore be precisely attached to the displacer and planar spring 20 within a limit of concentricity and perpendicularity in order for the displacer and piston not to become jammed during motion. A linear alternator 35 is conventionally attached to the piston 12. Because the piston and displacer move co-axially, there is an out-of-balance reaction force on the casing 10 that is conventionally balanced by a dynamic balancer 32 attached to the casing 10 for minimizing the axial vibrations that result from the axially reciprocating masses.
The well-known gamma configuration overcomes this alignment problem by arranging the displacer and piston in separate cylinders so that their individual requirements for precision do not interfere with each other as in the case of the beta configuration. However, a disadvantage of the gamma arrangement is that it has a higher dead volume than the beta configured machine. Further, in most prior art gamma machines, the placement of the piston and displacer in separate cylinders results in both an oscillating torque and a force on the casing that is more difficult to balance than the single oscillating axial force on the casing in the beta machine. This latter problem has been identified in at least one design published in the open literature where two opposing pistons are used to remove the oscillating torque component on the casing.
A second problem associated with beta free-piston machines is that the dynamic balancing technique that is universally used relegates these machines to operation at a single frequency. Arranging single frequency operation for engines is difficult and requires that the machine be frequency stabilized by, for example, direct electrical grid connection. On coolers, single frequency operation is easily established since the machines are electrically driven. However, even on these machines, there is sometimes a thermodynamic advantage in changing the operating frequency, which is not possible if a dynamic balancer is used. An ideal configuration for a free-piston Stirling machine would have:
a. No more precision than required for good thermodynamic operation.
b. A minimum dead volume.
c. Balancing under all operating conditions including different operating frequencies.
It is therefore an object and feature of the invention to provide a free piston Stirling machine in a gamma configuration that has power pistons with masses and orientations for balancing the vibration forces of the pistons and, most importantly, minimizes the dead (unswept) volume of the work space in order to reduce the size and mass of the machine and improve its efficiency.